The present invention relates to a thermoactuator provided in a wax-pellet thermostat for automotive engines and various thermo-sensors.
The thermo-actuator in the wax-pellet thermostat includes a steel rod, a guide member slidably engaged with the rod, a spool seal around the rod, a heat conductive cylinder secured to the guide member and exposed to a coolant, and wax pellets provided in the cylinder. The rod is sealed by a sealing device provided in the guide member.
When the temperature of the coolant rises in excess of a predetermined value of the thermostat, the wax expands. This forces the spool seal against the steel rod. As the wax squeezes the spool seal around the rod, the cylinder moves downward together with the guide member, thereby opening a valve. When the thermostat cools, the wax contracts, so that the spool seal expands.
On the other hand, the diaphragm portion of the spool seal at a central cylindrical portion has a relatively large thickness, for example, a thickness of 45% of the diameter of the rod. Accordingly, the restoring force of the spool seal to its original shape is very strong.
Accordingly, if the temperature of the coolant rapidly decreases from a high temperature (about 100.degree. C.) to a low temperature (below 40.degree. C.), the spool seal rapidly expands due to the restoring force of the spool seal to form a vacuum space between the spool seal and the rod. Thus, the coolant is inducted into the space by the vacuum.
Since the spool seal tightly grips the rod, the coolant in the space formed between the spool seal and the rod cannot be discharged. The valve is slightly raised by the coolant in the space, which causes the opening temperature of the valve to decrease. As a result, the engine is overcooled, thereby reducing the thermal efficiency of the engine and increasing the amount of emissions. The amount of the coolant in the space increases with time, so that the valve lift increases, which causes problems in the engine.